Base stations and mobile stations perform reverse power control to keep radio link performance stable while minimizing the influence of interference in a wireless communication system. The reverse power control is to control a reverse Carrier-to-Interference and Noise Ratio (CINR) required, keeping a target PER constant. Here, if the reverse target PER is set to a low value, a reliability of a radio link increases, while the reverse CINR required to keep the low target PER increases. Inversely, if the reverse target PER is set to a high value, the reliability of the radio link decreases, while the reverse CINR required decreases due to the high target PER. A variation of the required reverse CINR leads to a variation of an output of a mobile station (MS), causing a variation of reverse interference amount, capacity, and coverage of a base station (BS). That is, if the reverse CINR required is large, the mobile station output increases and the base station reverse interference amount increases and consequently, the base station reverse coverage and capacity decrease. Inversely, if the reverse CINR is small, the mobile station output decreases and thus, the base station reverse interference amount decreases and its reverse coverage and capacity increase; however, the capacity increases to some degree but decreases at any more degree. Accordingly, it can be said that there is a trade-off relationship between the reliability of the radio link guaranteed by the power control and the reverse capacity and coverage.
FIG. 1 is a flow diagram illustrating operation of a base station (BS) for increasing average capacity at a fixed target PER according to the conventional art.
Referring to FIG. 1, in step 100, the BS sets a specific channel and sets a target PER (Ptarget) maximizing system capacity among several target PERs. After that, the BS performs outer loop power control with reference to the set target PER (Ptarget) in step 102 and performs inner loop/closed loop power control in step 104 and load control in step 106. That is, the BS maintains the target PER (Ptarget) through the outer loop power control, then controls an output of a mobile station (MS) through the inner loop power control on the basis of the target PER, and then performs the load control considering interference.
However, if a fixed target PER is used in a wireless communication system like the conventional art, a BS cannot properly reflect a varying channel state because a channel varies according to time. For example, if a target PER is excessively high compared to an instantaneous channel, a CINR required decreases, an MS's output decreases, and an Interference-to-Noise Ratio (INR) decreases compared to a target INR, but a reliability of a radio link decreases, a PER increases, and system capacity decreases. Inversely, if the target PER is excessively low compared to the instantaneous channel, the CINR required increases and thus, the MS's output increases and the INR is higher than the target INR. As a result, a Modulation and Coding Scheme (MCS) level and an allocated resource decrease through load control, thereby causing a loss of system capacity.
Further, when Hybrid Automatic Repeat reQuest (HARQ) is used, an error can be restored through retransmission though a PER is generated. Thus, a system can operate power control at a higher target PER compared to when HARQ is not used. If power control is operated at a higher target PER, a reverse CINR required can decrease, MS's transmission power can decrease, and BS's coverage and capacity can increase. Also, if using the same transmission power, an MS can operate power control at a higher Modulation Product coding Rate (MPR), thus increasing a reverse throughput. But, if a target PER is fixed, a loss of HARQ gain is brought about because the target PER cannot increase despite its possible increase.
Thus, there is needed an apparatus and method for deciding an adaptive target PER maximizing system capacity in consideration of channel variation and HARQ.